1. Field of the Invention
The present invention relates to a valve control system for an internal combustion engine that is capable of selectively switching the operating mode of a valve system including intake valves and exhaust valves between a plurality of operating modes different in output characteristics.
2. Description of the Related Art
Conventionally, a valve control system of this kind has been proposed in the Publication of Japanese Patent No. 2619696. The internal combustion engine for which this valve control system is provided includes a variable valve timing mechanism that is capable of switching the valve timing (hereinafter referred to as “V/T”) of intake valves and/or exhaust valves between low-speed V/T suitable for low engine speed operation and high-speed V/T suitable for high engine speed operation. This variable V/T mechanism is of a hydraulic type that changes the V/T between the low-speed V/T and the high-speed V/T by supply and stoppage of hydraulic pressure by opening and closing a hydraulic pressure control valve provided in an oil passage.
This valve control system selects the low-speed V/T when the engine speed is lower than a first predetermined value on a low engine speed side, and the high-speed V/T when the engine speed is higher than a second predetermined value on a high engine speed side. Further, when the engine speed is between the first predetermined value and the second predetermined value, the V/T is switched at a time point the fuel injection amount set for the low-speed V/T and that set for the high-speed V/T become equal to each other. For example, when conditions for switching the V/T to the low-speed V/T are satisfied, the hydraulic pressure control valve is closed and the supply of the hydraulic pressure is stopped, and thereafter until the detected hydraulic pressure in the oil passage lowers to a predetermined pressure level, and further a predetermined time period elapses after the lowering of the hydraulic pressure to the predetermined pressure level, the fuel injection amount is held at a value for the high-speed V/T, and upon the lapse of the predetermined time period, the fuel injection amount is switched to a value for the low-speed V/T. Similarly, in switching the V/T to the high-speed V/T, after the hydraulic pressure starts to be supplied, when the detected hydraulic pressure in the oil passage rises to a predetermined pressure level, and further a predetermined time period elapses thereafter, the fuel injection amount is switched to a value suitable for the high-speed V/T. As described above, when switching the V/T, only after the lapse of the predetermined time period, the fuel injection amount suitable for a destination V/T is applied, whereby the fuel injection amount can be appropriately set while compensating for the delay in response to the control occurring before the switching of the variable V/T mechanism is actually completed after the hydraulic pressure control valve is closed or opened.
In the conventional valve control system described above, the supply and stoppage of the hydraulic pressure for switching the V/T is instantly carried out when the conditions therefor are satisfied. Therefore, when the engine is operated in a boundary area between respective operating regions for the two types of V/T, the frequency of switching between the supply and stoppage of the hydraulic pressure is increased, so that the driver feels the switching operation so frequent (busy) so that drivability is degraded. On the other hand, the fuel injection amount is not switched to the destination V/T before the lapse of the predetermined time period after the conditions for switching the V/T are satisfied. Therefore, when operating conditions of the engine, such as load thereon, are largely changed, e.g. when the demand for acceleration is high e.g. at the standing start of the vehicle, it takes longer time before the fuel injection amount is switched to a value suitable for the destination V/T, which impairs the feeling of acceleration to cause the driver to feel the operation of the vehicle tardy. This also prevents excellent drivability from being obtained. Further, there has been recently proposed a variable valve-actuating mechanism that is capable of switching the operating mode of the valve system between three or more modes. Particularly in such a variable valve-actuating mechanism, depending on operating conditions of the engine, there is a higher possibility of the switching of the operating mode being carried out at short intervals, making conspicuous the above-described problems.